bei48482_FM

AC

B D

The interference of water waves. Constructive interference occurs along the line

ABand destructive interference occurs along the line CD.

(a) b)

+= =

(

Figure 2.3(a) In constructive interference, superposed waves in phase reinforce each other. (b) In destructive

interference, waves out of phase partially or completely cancel each other.

When two or more trains of light waves meet in a region, they interfereto produce

a new wave there whose instantaneous amplitude is the sum of those of the original

waves. Constructive interference refers to the reinforcement of waves with the same

phase to produce a greater amplitude, and destructive interference refers to the partial

or complete cancellation of waves whose phases differ (Fig. 2.3). If the original waves

have different frequencies, the result will be a mixture of constructive and destructive

interference, as in Fig. 3.4.

The interference of light waves was first demonstrated in 1801 by Thomas Young,

who used a pair of slits illuminated by monochromatic light from a single source (Fig. 2.4).

From each slit secondary waves spread out as though originating at the slit; this is an ex-

ample of diffraction,which, like interference, is a characteristic wave phenomenon. Ow-

ing to interference, the screen is not evenly lit but shows a pattern of alternate bright

and dark lines. At those places on the screen where the path lengths from the two slits

differ by an odd number of half wavelengths (2, 32, 52,.. .), destructive inter-

ference occurs and a dark line is the result. At those places where the path lengths are

56 Chapter Two

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